The site would also collect identifying information about everyone who transacts while using it. Maybe phone number, or driver license photo. You are not allowed to have more than one account, although you can have multiple "personas" if you wish, each with their own ratings. (but creating multiple personas cannot fool site anti-abuse staff, who can all obviously see the same account behind it) Creating an account is hard. The site would employ advanced protections against account takeover.
Whenever you buy and sell from someone, perhaps you can see the "7 degrees of Kevin Bacon" style interpersonal connections that link you to them via vouches. If fraud happens, it makes all of your connections look bad, and there would be an incentive for them to shun you, if they care about retaining their access to the site. When fraudsters get onto the platform, you can always trace their vouches back to the "bad apple" who showed poor judgment and began inviting the wrong certain people. Indeed, perhaps in order to retain access to the platform overtime, you have to keep getting vouches every once in a while.
Big sites would not use these policies since they would invoice friction on purchases in result in lower conversion. But perhaps there is a niche somewhere for a highly trusted online auction house.
I wonder how far a system like this could go in stamping out fraud online. It seems like the problem with fraudsters is that they can always keep coming back with new accounts that everyone is forced to treat with the certain default degree of trust, and then the fraudsters exploit that. If you made it difficult for each person in the world to have more than one account on the service, then anyone engaging in repeat fraud would very quickly stand out as such. (As many personas as you want, but only one account)